Potable (drinking) water is commonly found in large commercial transport vehicles such as aircraft, railroad passenger cars, buses, boats and ships. Such water is placed on board these transport vehicles for use as drinking water, making ice cubes, culinary, cleaning and other sanitary purposes. Care must be taken to assure that harmful bacteria and organisms are not present and/or multiplying in the water found inside the potable water storage and distribution systems in these conveyances. Some organisms can live and multiply very rapidly under certain conditions. Most frequently cited to be of concern are coliform, E. coli, and legionella, but other common bacteria in domestic water supplies in the United States, Canada, and other countries include, but are not limited to, salmonella typhia, shingella sonnei, dysenteria, flexneri, boydii, vibro cholerae, campylobacter jejuni, yersinia enterocolitica, plesiomonas shigellojodes, and aeromonas hydrophila. Therefore, care is needed in the treatment and handling of water intended for such uses.
Within the United States, the Environmental Protection Agency has a responsibility for regulatory enforcement of the Safe Water Drinking Act of 1974. The Act covers, among other things, municipal drinking water supplies and other water supplies being used by the public. Other politically stable and economically developed countries have similar agencies with similar responsibilities. It is recognized by these agencies that it is economically impossible to provide absolutely pure water. There are, however, achievable, appropriate, and economical limits to the concentration of chemical, mineral, and organic contaminants. These contaminant concentrations are very low and generally do not represent a health threat to the general population.
Most of the watering points used to transfer water into the transport are filled using municipal, and otherwise regulated, treated public water supplies. Acceptably low concentrations of some offensive organisms are routinely introduced into watering points and consequently could find their way into the transport's potable water storage tank(s). Given the right conditions, these offensive organisms can grow and multiply and develop higher concentrations that can be harmful to otherwise strong and healthy people. These organisms can also form biofilms within watering points and a transport's potable water storage tank(s). Such biofilms can re-contaminate watering points and the transport's potable water storage tank(s) even after they have been emptied, flushed, and refilled.
While the cleaning system can be used in a variety of transports, the operating descriptions that follow will use transport aircraft for simplicity. Currently transport aircraft potable water systems are cleaned by following the guidelines established by the EPA. These guidelines have been adopted by large transport aircraft manufacturers. The cleaning procedures are incorporated in each individual aircraft's maintenance service bulletins and are typically accomplished as follows:
1. General                A. The passenger water system may be disinfected with an application of the following solutions and times:                    i) 50 parts per million of chlorine acidified with vinegar,            ii) 100 parts per million of chlorine—not acidified, and            iii) 50 parts per million of chlorine—not acidified.                            Note: Disinfecting time for A.i. and A.ii. is one hour.                                    Disinfecting time for A.iii. is four hours.                                                                                B. Recommended chlorine solution (concentrated). The following will result in a concentrated solution of 50 parts per million of acidified chlorine when added to a 40-gallon water tank. This concentrated solution minimizes the objectionable taste normally attributed to disinfectants and requires the shorter disinfecting time.                    i) 12 fluid ounces (354 ml) chlorine dioxide stabilized 2%.            ii) 12 fluid ounces (354 ml) acetic acid (vinegar), and            iii) Approximately 1 gallon (4 liters) clean water.                            Note: Let mixture stand 5 minutes to complete activation.                                                
2. Remove all filter cartridges (if so equipped) from water system, including coffeemakers, where applicable. Ensure filter caps are replaced.
Note: An excessive amount of disinfectant in the passenger water system may contaminate the filters and give an objectionable taste of chlorinated water. Use of the recommended chlorine solution (Ref. par. 1.B.) will minimize the objectionable taste.
3. Fill the system with chlorinated water which contains 50 or 100 parts per million of chlorine.                A. The water may be chlorinated in any of the following ways:                    i) A concentrated chlorine solution may be introduced first and then the system filled with drinkable water.            ii) The concentrated chlorine solution may be mixed with the drinkable water first; then pumped into the system.            iii) The concentrated solution may be added while the system is being filled.                        
4. After the system is filled with chlorinated water and chlorinated water has appeared at the overflow port, close the fill and overflow valve.
5. Pressurize the system and open each faucet, drinking fountain (if so equipped) and galley station until chlorinated water appears at each.
6. On airplanes with turn-to-flow faucets:                A. Close lavatory shutoff valves and open drain vent valves.        B. Open lavatory faucets and drinking fountains to allow water to flow through vent lines.        
7. Depressurize the system, open fill and overflow valve slowly, refill the tank with chlorinated water and close the fill and overflow valve.
8. Let chlorinated water stand in system for 1 hour if filled with 50 parts per million of acidified chlorine or 100 parts per million of chlorine (not acidified) solution. Let chlorinated water stand for 4 hours if filled with 50 parts per million of chlorine (not acidified).
9. Open tank drain valve, vent valves (airplanes with turn-to-flow faucets), lavatory drain valves and lavatory shutoff valves.
10. When water stops flowing from drain outlets, close drain valves and vent valves (airplanes with turn-to-flow faucets).
11. Refill tank with drinkable water and pressurize system.
12. Open each faucet, drinking fountain (if so equipped) and galley station.
13. On airplanes with turn-to-flow faucets, repeat steps in 6 and then open lavatory shutoff valves.
14. Continue to flush system with drinkable water until chlorine content is not objectionable.
15. Install new filter cartridges (if so equipped) and coffeemaker if applicable.